System for accomplishing bi-directional audio data and control communications

ABSTRACT

A system for accomplishing bi-directional digital audio data and control communications. There is a host end that includes a host transceiver, a host digital signal processor that outputs a master clock signal and audio data, and a source of low-voltage power for the host components. There are a number of remote nodes connected together serially. Each remote node includes a transceiver and a low-voltage power supply that provides power to the other remote node components. A shielded two-wire communication network connects the host node to one of the remote nodes, and connects each remote node to either one or two other remote nodes in a daisy-chain configuration. The host end is enabled to transmit and receive over the communication network digital audio data and digital control signals. The remote nodes are each enabled to receive over the communication network digital audio data and digital control signals, and are each enabled to transmit over the communication network digital control signals. The source of low-voltage power at the host end is coupled to the communication network. The remote power supplies each have an input coupled to the communication network to derive the power for the remote nodes from the power coupled to the network by the host end.

FIELD

This disclosure relates to bi-directional digital audio data and controlcommunications.

BACKGROUND OF THE INVENTION

Motor vehicle audio systems and audio networks in motor vehicleinteriors are becoming increasingly more complex. For example, eachpassenger space may be provided with its own audio capabilities.Further, microphones are sometimes used in vehicle cabins for purposessuch as speech recognition and active noise reduction. Motor vehicleaudio systems may require one to several microphones and/or an amplifierat each passenger location. These peripheral or remote nodes typicallyreceive audio data from (and transmit audio to, when microphones areused) an audio system processing unit, such as a head unit or a separateprocessing and amplification unit, that is typically located elsewherein the vehicle such as under the dash or in the trunk area. The resultis that the complexity, data transfer needs and wiring requirements forvehicle audio systems have become unwieldy. For example, the audionetwork typically requires three wires between each microphone and theaudio processing unit. The wiring alone consumes substantial valuablevehicle interior space and weight that could otherwise be devoted toother important aspects such as passenger room and comfort. Further, theaudio data and control signal communication requires a complex,expensive audio data and control communications system.

SUMMARY

In general, one aspect of the invention features a system foraccomplishing bi-directional digital audio data and controlcommunications. There is a host end that includes a host transceiver, ahost digital signal processor that outputs a master clock signal andaudio data, and a source of low-voltage power for the host components.There are a number of remote nodes connected together serially. Eachremote node includes a transceiver and a low-voltage power supply thatprovides power to the other remote node components. A two-wirecommunication network (e.g., a shielded twisted pair) connects the hostnode to one of the remote nodes, and connects each remote node to eitherone or two other remote nodes in a daisy-chain configuration. The hostend is enabled to transmit and receive over the communication networkdigital audio data and digital control signals. The remote nodes areeach enabled to receive over the communication network digital audiodata and digital control signals, and are each enabled to transmit overthe communication network digital control signals. The source oflow-voltage power at the host end is coupled to the communicationnetwork. The remote power supplies each have an input coupled to thecommunication network to derive the power for the remote nodes from thepower coupled to the network by the host end.

Various implementations of the invention may include one or more of thefollowing features. The host may include a low-voltage power supply thatis the source of low-voltage power. The remote nodes may includeamplifiers. The communication network may carry low voltage differentialsignaling (LVDS) signals. The remote nodes may include a first LVDSdriver/receiver and a second LVDS driver/receiver. The control signalsmay be transmitted along with the audio data. The audio data and thecontrol signals may be transmitted in discrete data frames. These dataframes may include forward data sent from the host end and return datasent from a remote node. The remote nodes may each be assigned a regularframe slot such that they are adapted to send frames to the host node atregular intervals.

Various additional implementations of the invention may include one ormore of the following features. The source of low-voltage power for thehost components may be a power supply that provides power at a voltageof below 6.5V. The remote nodes may include a remote microcontroller.The remote components may include an audio sink such as a loudspeakerand/or an audio source such as microphone (which may be digitalmicrophone) or a line input. The remote nodes may further include asub-system that extracts (derives or recovers) the master clock signalfrom received digital audio data. This sub-system may include aphase-locked loop (PLL) that is locked to a PLL input signal and outputsa remote node clock signal. The sub-system can also include a switchthat selects the PLL input signal. The switch may select from thederived clock signal and the remote node clock signal. The switch mayselect the derived clock signal while forward data is being received bythe remote node, and select the remote node clock signal while returndata is being sent by the remote node.

In general, in another aspect the invention features a system foraccomplishing bi-directional digital audio data and controlcommunications in a vehicle interior. The system has a host end thatincludes a transceiver, a digital signal processor that outputs a masterclock signal and audio data, and a low-voltage power supply. There are anumber of remote nodes connected together serially. Each remote nodeincludes a transceiver, first and second low voltage differentialsignaling (LVDS) drivers/receivers, and a low-voltage power supply thatprovides power to the other remote node components. There is a shieldedtwo-wire communication network that carries LVDS signals and connectsthe host end to one of the remote nodes and each remote node to eitherone or two other remote nodes in a daisy-chain configuration. The hostend is enabled to transmit and receive over the communication networkdigital audio data and digital control signals. The audio data and thecontrol signals are transmitted together in discrete data frames thateach include forward data sent from the host end and return data sentfrom a remote node. The remote nodes are each assigned a regular frameslot such that they send return data to the host transceiver at regularintervals. The source of low-voltage power at the host end is coupled tothe communication network via a signal isolator, and the remote powersupplies each have an input coupled to the communication network via asignal isolator to derive the power for the remote nodes from the powercoupled to the network by the host end. The shield acts as a commoncurrent return for the power. The remote components may further includea sub-system that extracts (derives or recovers) the master clock signalfrom received digital audio data. This sub-system can include a PLL thatis locked to a derived/recovered clock signal and outputs a remote nodeclock signal, and a switch that selects the PLL input signal from thederived clock signal and the remote node clock signal. The digital audiodata and digital control signals are preferably transmitted in discretedata frames that define forward data sent from the host end and returndata sent from a remote node. The switch may select the derived clocksignal while forward data is being received by the remote node andselect the remote node clock signal while return data is being sent bythe remote node.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a system for accomplishingbi-directional digital audio data and control communications;

FIG. 2 is a more detailed diagram of portions of an embodiment of asystem for accomplishing bi-directional digital audio data and controlcommunications that includes remote microphones;

FIG. 3 is a schematic block diagram of a sub-system that can be used toachieve synchronous bi-directional communication; and

FIG. 4 is a block diagram of a vehicle interior illustrating oneapplication of the system for accomplishing bi-directional digital audiodata and control communications.

DETAILED DESCRIPTION

This disclosure is related to a bi-directional digital audio data andcontrol communications system. The system can be used in situations inwhich audio systems require or could benefit from bi-directional flow ofaudio data and/or control signals. One example is a vehicle audiosystem. Another example is a telephone conferencing system in which anumber of microphones and/or speakers are in use.

The system may have an amplifier at the host end, which may be at theaudio head unit or a separate audio processing unit for a motor vehiclein the embodiment for use in motor vehicles. A number of remote nodesare supported. For use in a motor vehicle, the remote nodes can be butneed not be associated with a passenger location. The remote nodes caninclude an amplifier and/or one or more microphones, or other devicesthat act as audio sources or sinks. Microphones may be situated suchthat they sense sound at particular locations in a vehicle cabin. Incertain non-limiting examples, the system can be used as part of a cabinvoice recognition system, a cabin active noise reduction system, a cabinspeech/conversation enhancement system and/or an audio system withseparate amplifiers and speakers for a number of passenger locations.

FIG. 1 shows system 10 for accomplishing bi-directional digital audiodata and control communications. In one non-limiting embodiment, system10 may be used as part of the audio system for a vehicle interior.System 10 includes a host end 12 that communicates with a plurality ofremote nodes 20, 30 and 40. Host 12 may comprise an amplifier. System 10is adapted and arranged to communicate digital audio data and digitalcontrol and status communications in both directions between host 12 andeach of the remote nodes. The remote nodes may include one or more audiosources and/or sinks, such as microphones, amplifiers, or other devices.

The communications network that interconnects the components comprisesshielded two-wire network wiring that includes portion 14 that connectshost 12 to remote node 20. The remote nodes are connected in a daisychain configuration in which each node is connected to one or two othernodes as indicated by shielded two-wire bi-directional network wiringportions 24 and 34. Remote node 40 could also be connected to remotenode 20 for bi-directional communication. The network is configured andarranged to communicate digital audio data and/or digital controlsignals in both directions between the host end and the remote nodes.Each remote node is dynamically provided with a unique address, suchthat all hardware nodes can be built the same. Each remote node includesa transceiver. As a result, data and control signals initiated by host12 can be passed through one or more of the remote nodes before reachingtheir destination node. Likewise, audio data (e.g., microphone output orline input), and control communications such as status and error signalscan be communicated from any one of the remote nodes to host 12 alongthe daisy chain configuration, whether directly or through one or moreintervening remote nodes.

The arrangement and functionality of the components of system 10 allowsa vehicle audio system to be accomplished with only a shielded twistedpair of wires as the network wiring. The digital signaling (both theaudio data and any control signals) transmitted over the two-wirenetwork preferably is transmitted using low voltage differentialsignaling (LVDS). LVDS supports high speed digital signaling over atwisted pair. In the present system, these same wires carry low-voltageDC power superimposed on the data transmission lines. Thus the powersupply and data network is accomplished with a single shielded twistedpair. Further, the daisy chain configuration means that there is only asingle shielded twisted pair of wires between the audio head unit andone of the remote nodes (which can be the physically closest remotenode), and a single shielded twisted pair of wires between each of theremote nodes and one or two other remote nodes. The wiring of the systemthus occupies minimal space in the vehicle body, and the cost and weightof the wiring is substantially reduced. Further, the two-wire networkallows the use of smaller and less expensive components withsubstantially reduced pin counts (only three pins are needed for theshielded twisted pair at each termination) which also decreases thephysical space that needs to be devoted to the system: this isparticularly meaningful for remote nodes that are located in a vehiclecabin. For example, regardless of the number of microphones or othersinks or sources used in the system, the host amplifier only needs threepins to terminate the network. In contrast, existing systems need two orthree terminations per microphone. The system thus results in a hostamplifier pin count reduction of at least (N×2)−3, where N equals thenumber of microphones. System 10 supports the use of analog or digitalmicrophones as part of one or more of the remote nodes. When digitalmicrophones are used, the node does not require an analog to digitalconverter, which further decreases the space requirements and cost.Also, the host amplifier can be the single access point to one or morehigh level networks at the same time (such as CAN, MOST, E-Net, etc.)without the need for the remote nodes to duplicate all the connectionsand associated hardware needed for high-level networks. The system thusaccomplishes cost effective distributed amplifier systems with minimalnetwork overhead.

FIG. 2 is a more detailed diagram of portions of one embodiment ofsystem for accomplishing bi-directional digital audio data and controlcommunications 10 a. In this embodiment, host end 12 a includestransceiver 15, digital signal processor and microcontroller 16, andLVDS driver/receiver 17. The power for the remote nodes is provided overthe two-wire bi-directional network comprised of shielded twisted pair14 a that connects host end 12 a to remote node 20 a, and shieldedtwisted pair 24 a that connects remote node 20 a to the next node in thedaisy chain, and further shielded twisted pair connections betweenadditional remote nodes that can be included in the system but are notshown in FIG. 2 (the shielding is not shown in the drawing for the sakeof clarity). Each remote node (e.g., 20 a) includes a first LVDSdriver/receiver 21, a transceiver 22 and a second LVDS driver/receiver23. All of the remote nodes can be built with the same hardware, whichfurther simplifies the system design. The remote nodes may also includean amplifier and/or one or more analog or digital audio sinks orsources. Embodiment 10 a illustrates one non-limiting embodiment inwhich remote node 20 a includes digital microphone 25. As the network isextremely simple, small, low-cost microphones can be used. If themicrophone or other source is digital, the node does not require ananalog to digital converter. When an analog microphone or other analogsource or sink is present, an appropriate analog to digital converter(or D/A converter as appropriate) would be included in such remote node.

The transceivers can be accomplished with a field programmable gatearray (FPGA) or an ASIC or custom IC. The LVDS drivers/receivers can beaccomplished with an LVDS interface such as the National Semiconductor™DS90LV019. Alternatively, an ASIC or custom IC can accomplish thefunctionality of both the transceiver and the LVDS driver/receiver. DSP16 can be the source of the audio data if the network involves audiodata flow from the host amplifier (node) to one or more remote nodes;the same goes for control data. The DSP can also accept audio data fromone or more remote nodes. The host can also receive control data. Theremote node may have to request control data first or receive aninterrupt via the GPIO pin(s) that are used for this purpose. The GPIOpins can be used for simple point to point communication. An advantageis the low latency through the links, e.g., 1 frame time-length,1/44100=22 us. This makes such pins especially suitable for fast errorsignaling and interrupts to the host system. The use of interrupts forI2C style communication is not required but an option. The power supplyon the host powers the DSP, micro-controller, LVDS driver/receiver andall other components of the host. The remote nodes are also suppliedwith power from the host amplifier power supply, as further explainedbelow.

Audio functionality can be present in one or more of the remote nodes.This functionality can be any audio source(s) or sink(s); audioamplifiers and microphones are specific non-limiting examples of such.Another example of a source is a ⅛″ jack adapter at a remote node thatallows the connection of a personal audio device, such as an iPhone®from Apple Computer.

Each of the remote nodes derives its power from the shielded two-wirecommunication network. This can be accomplished in the embodiment bycoupling host end low voltage (e.g., lower than 6.5V) power supply 18 totwisted pair 14 a via signal isolator 19. Remote node power supply 27 iscoupled to the two-wire network via signal isolator 26 so that theremote node derives all of its power from the network. The host powersupply is thus able to provide low voltage power to all of thecomponents of all of the remote nodes. The shielding for the twistedpair is used as the common current return for the supply that issymmetrically transported over the two LVDS wires. Since the LVDSsignals operate at a high speed, an impedance network or filter isemployed to allow DC current to flow through while at the same timeallowing the LVDS drivers to signal as if the DC was not present. Inorder to preserve common mode rejection on the LVDS data, the filters(e.g., signal isolators 19 and 26) are symmetric. Current‘conventional/standard’ microphone solutions, utilizing electret biasedmicrophones, require at least 8-9V in bias voltage to operate properly.Start/Stop-type vehicles crank the engine every time the car startsmoving. An engine start causes a voltage dip on the battery power bus,typically from 12V down to about 6.5V, which is insufficient forconventional microphone systems. The system working at less than 6.5Vcan use MEMS digital microphones which can operate down to 1.8V. Hence,voltage dropouts as a result of start/stop functionality are not anissue in the system.

The network and components are preferably configured and adapted tocommunicate digital audio data and digital control communications inboth directions. The communication scheme supports the daisy-chainarrangement. Accordingly, each node has a unique network address.Further, the communication scheme accounts for audio data and controlsignals being transmitted from host end 12 a to any of the remote nodes,and audio data (if present), control signals and any other signals suchas error signals being communicated back from any or all of the remotenodes to the host end. When a remote node contains an audio sink and notan audio source, for example when it contains an amplifier chip, therewill be no audio data transported back from this node to the hostamplifier. However, if the remote node contains an audio source (such asa microphone, line input, etc.) then audio may be transported back tothe host, which happens at the same time as transporting audio from thehost to the remote node.

The bi-directional digital audio data and control signal communicationcan be accomplished in one non-limiting embodiment by transmitting theaudio data and control signals in discrete frames. The data framebetween nodes can be built up out of a ‘forward portion’ and a ‘returnportion.’ Each portion can include one or more sync bits, one or moreaudio bits, one or more control bits and one or more CRC bits. In onenon-limiting example, the frame size is fixed. A number of bits of eachframe are reserved for audio data. A number of additional bits of eachframe are reserved for control signals and/or error signals. Also, thedigital control element of the connection is an implicit form ofdiagnostics, so no external multiplexing and sensing is needed. Theembedded bidirectional audio and control element of the connectionallows for full remote node diagnostics and control, which has not beenaccomplished to date without the use of an expensive media network suchas MOST, Ethernet or IEEE 1394.

In one non-limiting example, each frame (or each forward and returnframe portion) can include 32 bit slots. Typically 20 or 24 bits areneeded for digital audio data. This leaves a number of bits that can bedevoted to control and error signals. Time division multiplexing can beused. Alternate frames (or, different portions of each frame) can travelin opposite directions, from and to the host end. Each of the remotenodes can be assigned a particular time slot, to allow forbi-directional communications between the host end and each of theremote nodes. As an alternative to TDM, the link can be run at a higherrate than the data clock to provide time for control and error signalcommunication.

The network timing is preferably derived from a master clock signal thatis outputted by DSP 16 or any other clock reference. DSP 16 typicallyinputs and outputs the audio data as multiple parallel streams of TDMdata, which is then serialized by the host transceiver. Audio data andother signals such as control and error signals are encoded via anyappropriate technique, one non-limiting example being Manchesterencoding. Manchester encoding allows the master clock signal to bederived from the encoded data by each of the remote nodes. In onenon-limiting embodiment the control signals are inter-integrated circuit(I2C) communications. In one non-limiting embodiment the transceiverseach define general purpose input/output (GPIO) pins and there is a 1:1pin correspondence between the host and each of the remote nodes, ineither direction. The pins can be hard-coded or configurable from thehost.

In one non-limiting embodiment, the remote transceivers can accomplishthe power control and sequencing required by the system foraccomplishing the bi-directional digital audio data and controlcommunications and power delivery over the twisted pair. Thetransceivers can be accomplished with an FPGA such as the IGLOO® or theProASIC®3 Nano available from Actel Corporation, Mountain View Calif. Ifthe remote transceivers cannot handle this functionality, a smallmicrocontroller (not shown) may be included in the remote nodes. In thiscase, the remote transceiver is a local slave to the microcontrollermaster. In the I2C scheme, the host microcontroller is always the masterand therefore the remote transceivers are each slaves, but the slavesare ‘proxy masters.’ When a microcontroller is present in the remotenodes and the remote transceiver is an I2C slave the registers of theremote transceivers are read locally as well as over the digital link bythe host. To support data transmission from the remote nodes to thehost, which happens asynchronously relative to the digital link andaudio clocks, RAM buffering (not shown) may need to be included in theremote nodes as a means to collect message information before it istransported.

In one non-limiting embodiment, remote node sub-system 50, FIG. 3, isused to help achieve synchronous bi-directional communication. Each dataframe includes a forward data portion in which data is sent from thehost to a remote node, and a return data portion in which data isreturned from that remote node to the host. During the forward portiondata is sent between the host or previous remote node and the nextremote node. The next remote node receives this data into LVDSdriver/receiver 52 and extracts (derives) the clock information from thedata using clock extract 54. PLL 58 is then locked to the extractedclock information. As a result, the two communicating nodes operatesynchronously. During the return data portion in which data is sent backfrom the receiving node, the node that is sending data would lose itsclock reference as it is no longer receiving data from which it derivesthe clock. PLL 58 for this purpose is first locked to itself: itdetaches from the input data stream and feeds the clock reference itproduces itself to its own input, as controlled by selection switchfunctionality 56. The control of switching functionality 56 can beaccomplished with a counter that runs within the transceiver. Thiscounter can be set for a fixed amount of counts within the frame toreach the direction switch points. This counter can be configurable toaccommodate changes in the size of the forward and backward channeldata. For a short duration of time, the clock rate produced by the PLLwill stay very close to the network rate, and the system will use thePLL for this short duration to signal data back to the receiving nodenear synchronously without any data corruption. If for some reason theportion of return data is too long for the PLL to remain stable enoughduring the return communication, an external PLL can be used, or a localoscillator could be included in order to achieve synchronous operation.

FIG. 4 is a highly schematic view of interior 82 of vehicle 80. Interior82 includes a number of passenger seating locations (e.g., seats 83-86).Host end 90 of system 100 communicates digitally over a shielded twistedpair connected between it and remote nodes 91-94, each physicallyassociated with one of locations 83-86. For example, host 90communicates directly with node 91 over twisted pair 101, and nodes91-94 are interconnected in a daisy-chain configuration via twistedpairs 103-105. Remote nodes 91-94 can include an amplifier or otheranalog or digital audio sink and/or one or more analog or digitalmicrophones or other audio sources. This system enables remote nodesthat are physically small and relatively inexpensive and requires only asingle shielded twisted pair to accomplish the network that connects thecomponents in a daisy chain configuration, and with remote node powerprovided by the host. System 100 can thus be used to support vehicleinterior audio system functionalities, including but not limited toseparate listening zones, separate speech enhancement zones and/oractive noise reduction in separate zones, e.g., one zone for eachoccupant location.

Other implementations are within the scope of the following claims.

The invention claimed is:
 1. A system for accomplishing bi-directionaldigital audio data and control communications, comprising: a host endcomprising host components that comprise a host transceiver, a hostdigital signal processor that outputs a master clock signal and audiodata, and a source of low-voltage power for the host components; aplurality of remote nodes connected together serially, each remote nodecomprising remote components that comprise a remote transceiver, and aremote low-voltage power supply that provides power to the other remotenode components; wherein the remote components further comprise asub-system that derives the master clock signal from received digitalaudio data, wherein the sub-system comprises a phase-locked loop (PLL)that is locked to a PLL input signal and outputs a remote node clocksignal, wherein the sub-system further comprises a switch that selectsthe PLL input signal and wherein the switch selects from the derivedclock signal and the remote node clock signal; a two-wire communicationnetwork that connects the host end to one of the remote nodes, andconnects each remote node to either one or two other remote nodes in adaisy-chain configuration; wherein the host end is enabled to transmitand receive over the communication network digital audio data anddigital control signals; wherein the remote nodes are each enabled toreceive over the communication network digital audio data and digitalcontrol signals, and are each enabled to transmit over the communicationnetwork digital control signals; and wherein the source of low-voltagepower at the host end is coupled to the communication network, and theremote power supplies each have an input coupled to the communicationnetwork to derive the power for the remote nodes from the power coupledto the network by the host end.
 2. The system of claim 1 wherein thehost components further comprise a low-voltage power supply that is thesource of low-voltage power.
 3. The system of claim 1 wherein the remotenodes comprise amplifiers.
 4. The system of claim 1 wherein the remotecomponents further comprise one or more audio sources.
 5. The system ofclaim 4 wherein the audio sources comprise one or more microphones. 6.The system of claim 5 wherein the microphones are digital.
 7. The systemof claim 1 wherein the communication network carries low voltagedifferential signaling (LVDS) signals.
 8. The system of claim 7 whereinthe control signals are transmitted along with the audio data.
 9. Thesystem of claim 8 wherein the audio data and the control signals aretransmitted in discrete data frames that each comprise a plurality ofbits.
 10. The system of claim 9 wherein the remote nodes are eachassigned a regular frame slot such that they are adapted to send framesto the host transceiver at regular intervals.
 11. The system of claim 7wherein the remote components further comprise a first LVDSdriver/receiver and a second LVDS driver/receiver.
 12. The system ofclaim 1 wherein the source of low-voltage power for the host componentscomprises a power supply that provides power at a voltage of less than6.5V.
 13. The system of claim 1 wherein the remote nodes each furthercomprise a remote microcontroller.
 14. The system of claim 1 wherein theremote components further comprise one or more audio sinks.
 15. Thesystem of claim 14 wherein the audio sinks comprise one or morespeakers.
 16. The system of claim 1 wherein the digital audio data anddigital control signals are transmitted in discrete data frames, and thedata frames comprise forward data sent from the host end and return datasent from a remote node.
 17. The system of claim 16 wherein the switchselects the derived clock signal while forward data is being received bythe remote node, and selects the remote node clock signal while returndata is being sent by the remote node.
 18. The system of claim 1 whereinthe digital audio data and digital control signals are transmitted indiscrete data frames, and the data frames comprise forward data sentfrom the host end and return data sent from a remote node.
 19. A systemfor accomplishing bi-directional digital audio data and controlcommunications in a vehicle interior, comprising: a host end comprisinghost components that comprise a host transceiver, a host digital signalprocessor that outputs a master clock signal and audio data, and alow-voltage power supply; a plurality of remote nodes connected togetherserially, each remote node comprising remote components that comprise aremote transceiver, first and second low voltage differential signaling(LVDS) drivers/receivers, and a remote low-voltage power supply thatprovides power to the other remote node components; a shielded two-wirecommunication network that carries LVDS signals and connects the hostend to one of the remote nodes and each remote node to either one or twoother remote nodes in a daisy-chain configuration; wherein the host endis enabled to transmit and receive over the communication networkdigital audio data and digital control signals, wherein the audio dataand the control signals are transmitted together in discrete data framesthat each comprise forward data sent from the host end and return datasent from a remote node, and wherein the remote nodes are each assigneda regular frame slot such that they send return data to the hosttransceiver at regular intervals; and wherein the source of low-voltagepower at the host end is coupled to the communication network via asignal isolator, the remote power supplies each have an input coupled tothe communication network via a signal isolator to derive the power forthe remote nodes from the power coupled to the network by the host end,and the shield acts as a common current return for the power.
 20. Thesystem of claim 19 wherein the remote components further comprise asub-system that derives the master clock signal from received digitalaudio data, the sub-system comprising a phase locked loop (PLL) that islocked to a PLL input signal and outputs a remote node clock signal, anda switch that selects the PLL input signal from the derived clock signaland the remote node clock signal; and wherein the digital audio data anddigital control signals are transmitted in discrete data frames thatcomprise forward data sent from the host end and return data sent from aremote node, and the switch selects the derived clock signal whileforward data is being received by the remote node and selects the remotenode clock signal while return data is being sent by the remote node.